During normal operation of a rotorcraft having a plurality of rotor blades connected to a rotor hub, the revolving rotor blades experience aerodynamic asymmetries which subject the rotor blades to alternating loads. These alternating loads impose a forcing frequency onto the rotor hub and rotorcraft structure, defined as a function of the number of rotor blades and their rotational speed, thereby causing the rotorcraft structure to vibrate at substantially the same frequency.
Since this rotor-induced vibration typically occurs at a constant frequency, a passive resonant-type vibration absorber is frequently utilized to generate opposing vibratory loads in the rotorcraft structure. Such an absorber is typically a single degree-of-freedom mass-spring system, attached to the rotorcraft's structure at a specific location, and designed to vibrate at its natural frequency in response to the rotor-induced vibration in the structure. When the rotorcraft structure causes the absorber to vibrate at its natural frequency, the reaction forces exerted by the absorber on the structure are out-of-phase with the vibratory forces experienced by the structure. Therefore, the vibration of the absorber has the effect of reducing the amplitude of the rotor-induced vibration at the point of connection between the absorber and the rotorcraft structure.
In U.S. Pat. No. 4,230,291, assigned to United Technologies Corp. (hereinafter "'291 patent"), a tuned spring-mass vibration absorber is disclosed for use in a rotorcraft. The vibration absorber in the '291 patent comprises a plurality of cantilevered leaf springs integrally connecting a dynamic mass to a vibrating support member. The leaf springs are connected to the dynamic mass at selected stations such that the mass center of gravity and the spring center of force are coincident. The leaf springs are shaped such that they constitute a substantial part of the vibration absorber effective mass, and are pivotally connected to the support member so as not to impart any moments thereto.
The pivotal connections between the leaf springs and the support member are achieved through the use of spherical bearing members in combination with bolts, wherein this combination transfers vertical, lateral, and longitudinal shear forces from the leaf springs to the support member, but does not impose moments on the support member. A drawback to the vibration absorber disclosed in the '291 patent is that mislocation of the spherical bearing members can alter the boundary conditions of the vibration absorber, thereby decreasing the ability of the vibration absorber to properly absorb vibration.
Specifically, mislocation of the spherical bearing members, which can occur during initial installation of the vibration absorber, can induce radial and axial steady loads in the bearings, thereby increasing rotational resistance and changing the vibration absorber's natural frequency. This change in the natural frequency makes tuning of the vibration absorber dependent upon the installation conditions associated with individual rotorcraft. This rotorcraft dependence necessitates a dynamic re-tuning of the vibration absorber during operation of the rotorcraft such that the natural frequency of the vibration absorber equals the anticipated rotor-induced vibratory frequency. In addition, the bearing loads induced by mislocated spherical bearing members can be relieved over time due to frictional wear in the spherical bearing members, thereby once again causing a change in the vibration absorber's natural frequency and necessitating dynamic re-tuning.
In order to tune a typical spring-mass vibration absorber (such as the one disclosed in the '291 patent) to the proper natural frequency, a stack-up of heavy and light tuning weights are connected to the underside of the dynamic mass, thereby increasing the total weight of the dynamic mass. Among the drawbacks to tuning a vibration absorber in this way is that in a typical installation setting for a vibration absorber, the structural design of the rotorcraft can present limitations as to the operational envelope of the vibration absorber. Specifically, during operation of the vibration absorber, a stack of multiple heavy and light tuning weights, in series, on the underside of the dynamic mass can cause contact between the tuning weights and nearby rotorcraft structure. In addition, since the tuning weights are stacked in series, it becomes more difficult to access any light or heavy tuning weights disposed distal from the bottom end of the stack.